1. Field of the Invention
The present invention relates to semiconductor device manufacture, and especially to a method of manufacturing a MIM (Metal Insulator Metal) capacitor element.
2. Description of the Background Art
Conventionally, a semiconductor device employs a capacitor element whose upper and lower electrodes are formed of polysilicon. In the process of manufacturing a semiconductor device having such a capacitor element, the upper electrode of the capacitor element is formed of the same layer of polysilicon as the gate electrode of a transistor. However in recent years, in the formation of the gate electrode of the transistor, ions are often implanted into polysilicon which forms the gate electrode. Thus, the upper electrode of the capacitor element is also formed of ion-implanted polysilicon. As a result, depletion occurs in the polysilicon of the upper electrode, which varies the capacitance value of the capacitor element depending on the applied voltage and the temperature. This causes the problem of a malfunction of the semiconductor device.
To solve such a problem, in recent years, a capacitor element having a MIM (Metal Insulator Metal) structure using metal as an electrode has been employed (see, for example, Japanese Patent Application Laid-open No. 1-198061 (1989) (p. 2, FIG. 1)). However, since a conventional MIM capacitor element has its upper and lower electrodes at different levels, it is difficult to form a contact hole on each of the upper and lower electrodes after formation of the MIM capacitor element.
There is also a technique for incorporating a MIM capacitor element into a single layer of insulating film (see, for example, Japanese Patent Application Laid-open No. 2002-100680 (pp. 4-5, FIGS. 3-5)). According to the MIM capacitor element in this publication, the upper and lower electrodes are at the same level.
In the process of forming a MIM capacitor element according to Japanese Patent Application Laid-open No. 2002-100680, however, the lower electrode needs to be formed into a complicated shape, and also relatively high accuracy is required for the depth of a trench for forming the lower electrode and the thickness of a dielectric layer to be deposited. This can complicate the manufacturing process.
For semiconductor devices having a MIM capacitor element, forming through simple processes the MIM capacitor element and contacts to the upper and lower electrodes of the MIM capacitor element is an important issue for simplification of the semiconductor device manufacturing process.